States of matter

• Particle theory


  A model that explains the three states of matter by considering each particle as a small, solid, inelastic sphere
• Three states of matter

  • Solid
  • Liquid
  • Gas
• Solids


  • Strong forces of attraction between particles, which holds them close together in fixed positions to form a very regular lattice arrangement
  • Particles don't move from their positions, so all solids keep a definite shape and volume, and don't flow like liquids
  • Particles vibrate about their positions - the hotter the solid becomes, the more they vibrate (causing solids to expand slightly when heated)
• Liquids


  • Weak force of attraction between the particles, they're randomly arranged and free to move past each other, but they tend to stick closely together
  • Liquids have a definite volume but don't keep a definite shape, and will flow to fill the bottom of a container
  • Particles are constantly moving with random motion, the hotter the liquid gets, the faster they move (causing liquids to expand slightly when heated)
• Gases


  • Force of attraction between the particles is very weak, they're free to move and are far apart, the particles in gases travel in straight lines
  • Gases don't keep a definite shape or volume and will always fill any container
  • Particles move constantly with random motion, the hotter the gas gets, the faster they move (gases either expand when heated, or their pressure increases)
• Particle theory is a great model for explaining the three states of matter, but it isn't perfect. In reality, the particles aren't solid or inelastic and they aren't spheres - they're atoms, ions or molecules. Also, the model doesn't show the forces between the particles, so there's no way of knowing how strong they are.
• State symbols


  Tell you the state of a substance in an equation
• Physical changes


  1. Solid particles gain more energy when heated

  2. This makes the particles vibrate more, which weakens the forces that hold the solid together

  3. At the melting point, the particles have enough energy to break free from their positions, turning the solid into a liquid

  4. When a liquid is heated, the particles get even more energy, making them move faster and breaking the bonds holding the liquid together

  5. At the boiling point, the particles have enough energy to break their bonds, turning the liquid into a gas

  6. As a gas cools, the particles no longer have enough energy to overcome the forces of attraction between them, so bonds form and the gas becomes a liquid (condensing)
• The amount of energy needed for a substance to change state depends on how strong the forces between particles are. The stronger the forces, the more energy is needed to break them, and so the higher the melting and boiling points of the substance.
• If the temperature is below the melting point of a substance, it'll be a solid. If it's above the boiling point, it'll be a gas. If it's in between the two points, then it's a liquid.
• Molecular substances


  • Oxygen
  • Nitrogen
  • Bromine
• Oxygen and nitrogen have boiling points below 25 °C, so will both be gases at room temperature. Bromine melts at -7 °C and boils at 59 °C, so it'll be a liquid at room temperature.
• The bulk properties such as the melting point of a material depend on how lots of atoms interact together. An atom on its own doesn't have these properties.